Three dimensional structure memory

ABSTRACT

A Three-Dimensional Structure (3DS) Memory allows for physical separation of the memory circuits and the control logic circuit onto different layers such that each layer may be separately optimized. One control logic circuit suffices for several memory circuits, reducing cost. Fabrication of 3DS memory involves thinning of the memory circuit to less than 50 μm in thickness and bonding the circuit to a circuit stack while still in wafer substrate form. Fine-grain high density interlayer vertical bus connections are used. The 3DS memory manufacturing method enables several performance and physical size efficiencies, and is implemented with established semiconductor processing techniques.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to stacked integrated circuitmemory.

[0003] 2. State of the Art

[0004] Manufacturing methods for increasing the performance anddecreasing the cost of electronic circuits, nearly without exception,are methods that increase the integration of the circuit and decreaseits physical size per equivalent number of circuit devices such astransistors or capacitors. These methods have produced as of 1996microprocessors capable of over 100 million operations per second thatcost less than $1,000 and 64 Mbit DRAM circuits that access data in lessthan 50 ns and cost less than $50. The physical size of such circuits isless than 2 cm². Such manufacturing methods support to a large degreethe economic standard of living in the major industrialized countriesand will most certainly continue to have significant consequences in thedaily lives of people all over the world.

[0005] Circuit manufacturing methods take two primary forms: processintegration and assembly integration. Historically the line betweenthese two manufacturing disciplines has been clear, but recently withthe rise in the use of MCMs (Multi-Chip Modules) and flip-chip dieattach, this clear separation may soon disappear. (The predominate useof the term Integrated Circuit (IC) herein is in reference to anIntegrated Circuit in singulated die form as sawed from a circuitsubstrate such as s semiconductor wafer versus, for example, anIntegrated Circuit in packaged form.) The majority of ICs when ininitial die form are presently individually packaged, however, there isan increasing use of MCMs. Die in an MCM are normally attached to acircuit substrate in a planar fashion with conventional IC die I/Ointerconnect bonding methods such as wire bonding, DCA (Direct ChipAttach) or FCA (Flip-Chip Attach).

[0006] Integrated circuit memory such as DRAM, SRAM, flash EPROM,EEPROM, Ferroelectric, GMR (Giant MagnetoResistance), etc. have thecommon architectural or structural characteristic of being monolithicwith the control circuitry integrated on the same die with the memoryarray circuitry. This established (standard or conventional)architecture or circuit layout structure creates a design trade-offconstraint between control circuitry and memory array circuitry forlarge memory circuits. Reductions in the fabrication geometries ofmemory cell circuitry has resulted in denser and denser memory ICs,however, these higher memory densities have resulted in moresophisticated control circuitry at the expense of increased area of theIC. Increased IC area means at least higher fabrication costs per IC(fewer ICs per wafer) and lower IC yields (fewer working ICs per wafer),and in the worst case, an IC design that cannot be manufactured due toits non-competitive cost or unreliable operation.

[0007] As memory density increases and the individual memory cell sizedecreases more control circuitry is required. The control circuitry of amemory IC as a percentage of IC area in some cases such as DRAMsapproaches or exceeds 40%. One portion of the control circuitry is thesense amp which senses the state, potential or charge of a memory cellin the memory array circuitry during a read operation. The sense ampcircuitry is a significant portion of the control circuitry and it is aconstant challenge to the IC memory designer to improve sense ampsensitivity in order to sense ever smaller memory cells while preventingthe area used by the sense amp from becoming too large.

[0008] If this design constraint or trade-off between control and memorycircuits did not exist, the control circuitry could be made to performnumerous additional functions, such as sensing multiple storage statesper memory cell, faster memory access through larger more sensitivesense amps, caching, refresh, address translation, etc. But thistrade-off is the physical and economic reality for memory ICs as theyare presently made by all manufacturers.

[0009] The capacity of DRAM circuits increase by a factor of four fromone generation to the next; e.g. 1 Mbit, 4 Mbit, 16 Mbit and 64 MbitDRAMs. This four times increase in circuit memory capacity pergeneration has resulted in larger and larger DRAM circuit areas. Uponintroduction of a new DRAM generation the circuit yields are too lowand, therefore, not cost effective for high volume manufacture. It isnormally several years between the date prototype samples of a new DRAMgeneration are shown and the date such circuits are in volumeproduction.

[0010] Assembling die in a stacked or three dimensional (3D) manner isdisclosed in U.S. Pat. No. 5,354,695 of the present inventor,incorporated herein by reference. Furthermore, assembling die in a 3Dmanner has been attempted with regard to memory. Texas Instruments ofDallas Tex., Irvine Sensors of Costa Mesa Calif. and Cubic MemoryCorporation of Scotts Valley Calif., have all attempted to producestacked or 3D DRAM products. In all three cases, conventional DRAMcircuits in die form were stacked and the interconnect between each DRAMin the stack was formed along the outside surface of the circuit stack.These products have been available for the past several years and haveproved to be too expensive for commercial applications, but have foundsome use in space and military applications due to their small physicalsize or footprint.

[0011] The DRAM circuit type is referred to and often used as an examplein this specification, however, this invention is clearly not limited tothe DRAM type of circuit. Undoubtedly memory cell types such as EEPROMs(Electrically Erasable Programmable Read Only Memories), flash EPROM,Ferroelectric, or combinations (intra or inter) of such memory cells canalso be used with the present Three Dimensional Structure (3DS) methodsto form 3DS memory devices.

[0012] The present invention furthers, among others, the followingobjectives:

[0013] 1. Several-fold lower fabrication cost per megabyte of memorythan circuits conventionally made solely with monolithic circuitintegration methods.

[0014] 2. Several-fold higher performance than conventionally madememory circuits.

[0015] 3. Many-fold higher memory density per IC than conventionallymade memory circuits.

[0016] 4. Greater designer control of circuit area size, and therefore,cost.

[0017] 5. Circuit dynamic and static self-test of memory cells by aninternal controller.

[0018] 6. Dynamic error recovery and reconfiguration.

[0019] 7. Multi-level storage per memory cell.

[0020] 8. Virtual address translation, address windowing, variousaddress functions such as indirect addressing or content addressing,analog circuit functions and various graphics acceleration andmicroprocessor functions.

SUMMARY OF THE INVENTION

[0021] The present 3DS memory technology is a stacked or 3D circuitassembly technology. Features include:

[0022] 1. Physical separation of the memory circuits and the controllogic circuit onto different layers;

[0023] 2. The use of one control logic circuit for several memorycircuits;

[0024] 3. Thinning of the memory circuit to less than about 50 μm inthickness forming a substantially flexible substrate with planarprocessed bond surfaces and bonding the circuit to the circuit stackwhile still in wafer substrate form; and

[0025] 4. The use of fine-grain high density inter layer vertical busconnections.

[0026] The 3DS memory manufacturing method enables several performanceand physical size efficiencies, and is implemented with establishedsemiconductor processing techniques. Using the DRAM circuit as anexample, a 64 Mbit DRAM made with a 0.25 μm process could have a diesize of 84 mm², a memory area to die size ratio of 40% and a access timeof about 50 ns for 8 Mbytes of storage; a 3DS DRAM IC made with the same0.25 μm process would have a die size of 18.6 mm², use 17 DRAM arraycircuit layers, a memory area to die size ratio of 94.4% and an expectedaccess time of less than 10 ns for 64 Mbytes of storage. The 3DS DRAM ICmanufacturing method represents a scalable, many-fold reduction in thecost per megabyte versus that of conventional DRAM IC manufacturingmethods. In other words, the 3DS memory manufacturing method represents,at the infrastructure level, a fundamental cost savings that isindependent of the process fabrication technology used.

BRIEF DESCRIPTION OF THE DRAWING

[0027] The present invention may be further understood from thefollowing description in conjunction with the appended drawing. In thedrawing:

[0028]FIG. 1a is a pictorial view of a 3DS DRAM IC manufactured withMethod A or Method B and demonstrating the same physical appearance ofI/O bond pads as a conventional IC die;

[0029]FIG. 1b is a cross-sectional view of a 3DS memory IC showing themetal bonding interconnect between several thinned circuit layers;

[0030]FIG. 1c is a pictorial view of a 3DS DRAM IC stack bonded andinterconnected face-down onto a larger conventional IC or another 3DSIC;

[0031]FIG. 2a is a diagram showing the physical layout of a 3DS DRAMarray circuit block with one data-line set of bus lines, i.e one port;

[0032]FIG. 2b is a diagram showing the physical layout of a 3DS DRAMarray circuit block with two sets of data-line bus lines, i.e. twoports;

[0033]FIG. 2c is a diagram showing the physical layout of a portion ofan exemplary memory controller circuit;

[0034]FIG. 3 is a diagram showing the physical layout of a 3DS DRAMarray circuit showing partitions for sixty-four (64) 3DS DRAM arrayblocks;

[0035]FIG. 4 is a cross-sectional view of a generic 3DS verticalinterconnection or feed-through in a thinned substrate;

[0036]FIG. 5 is a diagram showing the layout of a 3DS memory multiplexerfor down-selecting gate-line read or write selection.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] Referring to FIG. 1a and FIG. 1b, the 3DS (Three DimensionalStructure) memory device 100 is a stack of integrated circuit layerswith fine-grain vertical interconnect between all circuit layers. Theterm fine-grain inter-layer vertical interconnect is used to meanelectrical conductors that pass through a circuit layer with or withoutan intervening device element and have a pitch of nominally less than100 μm and more typically less than 10 μm, but not limited to a pitch ofless than 2 μm, as best seen in FIG. 2a and FIG. 2b. The fine-graininter-layer vertical interconnect also functions to bond together thevarious circuit layers. As shown in FIG. 1b, although the bond andinterconnect layers 105 a, 105 b, etc., are preferably metal, othermaterial may also be used as described more fully hereinafter.

[0038] The pattern 107 a, 107 b, etc. in the bond and interconnectlayers 105 a, 105 b, etc. defines the vertical interconnect contactsbetween the integrated circuit layers and serves to electrically isolatethese contacts from each other and the remaining bond material; thispattern takes the form of either voids or dielectric filled spaces inthe bond layers.

[0039] The 3DS memory stack is typically organized as a controllercircuit 101 and some number of memory array circuit layers 103,typically between nine (9) and thirty-two (32), but there is noparticular limit to the number of layers. The controller circuit is ofnominal circuit thickness (typically 0.5 mm or greater), but each memoryarray circuit layer is a thinned and substantially flexible circuit withnet low stress, less than 50 μm and typically less than 10 μm inthickness. Conventional I/O bond pads are formed on a final memory arraycircuit layer for use with conventional packaging methods. Other metalpatterns may be used such as insertion interconnection (disclosed inU.S. Pat. Nos. 5,323,035 and 5,453,404 of the present inventor), DCA(Direct Chip Attach) or FCA (Flip-Chip Attach) methods.

[0040] Further, the fine grain inter-layer vertical interconnect can beused for direct singulated die bonding between a 3DS memory die and aconventional die (wherein the conventional die could be the controllercircuit as shown in FIG. 1c) or a 3DS memory die and another 3DS memorydie; it should be assumed that the areas (sizes) of the respective diceto be bonded together can vary and need not be the same. Referring moreparticularly to FIG. 1c, a 3DS DRAM IC stack 100 is bonded andinterconnected face-down onto a larger conventional IC or another 3DS IC107. Optionally the 3DS stack 100 can be composed of only DRAM arraycircuits with the DRAM controller circuitry as part of the larger die.If the DRAM controller circuitry is part of the larger die, thenfine-grain vertical bus interconnect would be required (at the face 109of the 3DS DRAM IC stack 100) to connect the 3DS DRAM array circuit tothe DRAM controller, otherwise larger grain conventional interconnectioncould be incorporated (patterned) into the planarized bond layer.

[0041] As shown in FIG. 3, each memory array circuit layer includes amemory array circuit 300 composed of memory array blocks 301 (nominallyless than 5 mm² in area) and each block is composed of memory cells (inmuch the same manner as the cell array of a DRAM or EEPROM circuit),busing electrodes, and—at the option of the designer—enabling gates forselection of specific rows or columns of the memory array. Thecontroller circuit is composed of sense amps, address, control and drivelogic that would normally be found at the periphery of a typical memorycircuit of monolithic design such as in a conventional DRAM.

[0042] Fine-grain busing vertically connects the controllerindependently to each memory array layer such that the controller canprovide drive (power) or enable signals to any one layer withoutaffecting the state of any of the other layers. This allows thecontroller to test, read or write independently each of the memorycircuit layers.

[0043]FIG. 2a and FIG. 2b show examples of layouts of possible blocks ofa memory array such as the block 301 of FIG. 3. Although only a portionof the block is shown, in the illustrated embodiment, the blocks exhibitbilateral symmetry such that the layout of the complete block may beascertained from the illustrated portion. Abbreviations “T”, “L”, and“TL” are used following various reference numerals to indicate “Top”,“Left” and “Top-Left,” respectively, implying corresponding elements notshown in the figure.

[0044] Referring to FIG. 2a, a core portion 200 of the block is composedof a “sea” of memory cells. Logically, the aggregation of memory cellsmay be subdivided into “macrocells” 201 each containing some number ofmemory cells, e.g. an 8×8 array of 64 memory cells. At the periphery ofthe core is formed fine-grain vertical interconnect comprisinginter-layer bond and bus contact metallizations 400, described ingreater detail hereinafter with reference to FIG. 4. The fine-grainvertical interconnect includes I/O power and ground bus lines 203TL,memory circuit layer selects 205T, memory macro cell column selects207T, data lines 209L, and gate-line multiplexer (“mux”) selects 209TL.Gate-line multiplexers 211T are, in the illustrated embodiment, 4:1multiplexers used to select one of four columns within an eight-widememory macro cell column. Corresponding bottom-side 4:1 multiplexerscombine with the topside multiplexers 211T to form equivalent 8:1multiplexers for selecting a single gate-line from aneight-gate-line-wide memory macro cell column.

[0045] One implementation of a 4:1 gate-line bus muliplexer 500 is shownin FIG. 5. Gate-line enables 209TL′ (formed in a Metal-1 layer, forexample) control transistors 501 a through 501 d, respectively. Coupledto the transistors are respective gate lines 503 a through 503 d. Alsopartly visible are gate-lines 505 a through 505 d which are coupled to acorresponding 4:1 multiplexer (not shown). When one of the gate-lineenables is active, the corresponding gate-line is coupled to an outputline 507 of the multiplexer (formed in a Metal-2 layer, for example).The output line is connected to one or more vertical bus connectsthrough a line 509 (formed in a Metal-3 layer and corresponding to metalcontact 400 of vertical bus interconnect, for example) and tungstenplugs 511 and 513. The tungsten plug 513 joins the line 509 to avertical interconnect (not shown).

[0046] Referring again to FIG. 2a, in the case of a memory circuitlayer, the layer may also include output line enables (gates) fromcontroller layer enable signals 205T, for which I/O enables (gates) 213may be provided.

[0047] Note that at the memory layer level, each memory block 301 iselectrically isolated from every other memory block 301. Accordingly,the yield probability for each memory block is independent.

[0048] Additional read/write ports can be added as can additionalgate-line vertical interconnections; additional vertical interconnectioncan be used in a redundant manner to improve vertical interconnectyield. The 3DS memory circuit can be designed to have one or more dataread and write bus port interconnections. Referring to FIG. 2b, a memoryblock 301′ is shown as having a port P₀, (209L) and a further port P₁(209L′). The only limitation on the number of vertical interconnectionsis the overhead such vertical interconnections impose on the cost of thecircuit The fine-grain vertical interconnect method allows thousands ofinterconnects per block at an increase in die area of only a fewpercent.

[0049] As an example, the overhead of the vertical interconnect shown inFIG. 2b for a DRAM memory block of 4 Mbits with two read/write ports andimplemented in 0.35 μm or 0.15 μm design rules consists of approximately5,000 connections and is less than 6% of the total area of the memoryarray block. Therefore, the vertical interconnect overhead for eachmemory array circuit layer in the 3DS DRAM circuit is less than 6%. Thisis significantly less than that presently experienced in monolithic DRAMcircuit designs where the percentage of non-memory cell area can exceed40%. In a completed 3DS DRAM circuit the percentage of non-memory cellarea is typically less than 10% of the total area of all circuits in thestacked structure.

[0050] The 3DS memory device decouples control functions that normallywould be found adjacent the memory cells of monolithic memory circuitsand segregates them to the controller circuit. The control functions,rather than occurring on each memory array layer as in conventionalmemory ICs, occur only once in the controller circuit. This creates aneconomy by which several memory array layers share the same controllerlogic, and therefore, lowers the net cost per memory cell by as much asa factor of two versus conventional memory design.

[0051] The segregation of the control functions to a separate controllercircuit allows more area for such functions (i.e., an area equal to thearea one or several of the memory array blocks). This physicalsegregation by function also allows fabrication process segregation ofthe two very different fabrication technologies used for the controllogic and the memory array, again realizing additional fabrication costsavings versus the more complicated combined logic/memory fabricationprocess used for conventional memory. The memory array can also befabricated in a process technology without consideration of the processrequirements of control logic functions. This results in the ability todesign higher performance controller functions at lower cost than is thecase with present memory circuits. Furthermore, the memory array circuitcan also be fabricated with fewer process steps and nominally reducememory circuit fabrication costs by 30% to 40% (e.g., in the case of aDRAM array, the process technology can be limited to NMOS or PMOStransistors versus CMOS).

[0052] Hence, although bonding of sufficiently planar surfaces of amemory controller substrate and a memory array substrate using thermaldiffusion metal bonding is preferred, in the broader aspects of thepresent invention, the invention contemplates bonding of separate memorycontroller and memory array substrates by any of various conventionalsurface bonding methods, such as anisotropically conductive epoxyadhesive, to form interconnects between the two to provide random accessdata storage.

[0053] Referring to FIG. 2c, the layout of a portion of an exemplarymemory controller circuit is shown. The inter-layer bond and bus contactmetallization has the same pattern as previously described in relationto FIG. 2a. Instead of a sea of memory cells, however, there is providedmemory controller circuitry including, for example, sense amps and dataline buffers 215. Because of the increased availability of die area,multi-level logic may be provided in conjunction with the sense amps anddata line buffers 215. Also shown are address decode, gate-line and DRAMlayer select logic 217, refresh and self-test logic 219, ECC logic 221,windowing logic 223, etc. Note that self-test logic, ECC logic, andwindowing logic are provided in addition to functions normally foundwithin a DRAM memory controller circuit. Depending on die size or thenumber of controller circuit layers used, any of numerous otherfunctions may also be provided including, for example, virtual memorymanagement, address functions such as indirect addressing or contentaddressing, data compression, data decompression, audio encoding, audiodecoding, video encoding, video decoding, voice recognition, handwritingrecognition, power management, database processing, graphicsacceleration functions, microprocessor functions (including adding amicroprocessor substrate), etc.

[0054] The size of the 3DS memory circuit die is not dependent on thepresent constraint of containing the necessary number of memory cellsand control function logic on one monolithic layer. This allows thecircuit designer to reduce the 3DS circuit die size or choose a die sizethat is more optimal for the yield of the circuit. 3DS memory circuitdie size is primarily a function of the size and number of memory arrayblocks and the number of memory array layers used to fabricate the final3DS memory circuit. (The yield of a nineteen (19) layer, 0.25 μm process3DS DRAM memory circuit may be shown to be greater than 90% as describedbelow.) This advantage of selecting the 3DS circuit die size enables anearlier first production use of a more advanced process technology thanwould normally be possible for conventional monolithic circuit designs.This, of course, implies additional cost reductions and greaterperformance over the conventional memory circuits.

[0055] 3DS Memory Device Fabrication Methods

[0056] There are two principal fabrication methods for 3DS memorycircuits. The two 3DS memory fabrication methods, however, have a commonobjective which is the thermal diffusion metal bonding (also referred toas thermal compression bonding) of a number of circuit substrates onto arigid supporting or common substrate which itself may optionally also bea circuit component layer.

[0057] The supporting or common substrate can be a standardsemiconductor wafer, a quartz wafer or a substrate of any materialcomposition that is compatible with the processing steps of the 3DScircuit, the operation of the circuit and the processing equipment used.The size and shape of the supporting substrate is a choice that bestoptimizes available manufacturing equipment and methods. Circuitsubstrates are bonded to the supporting substrate and then thinnedthrough various methods. Circuit substrates may be formed on standardsingle crystal semiconductor substrates or as polysilicon circuitsformed on an appropriate substrate such as silicon or quartz.

[0058] Polysilicon transistor circuits have the important cost savingoption of incorporating a parting layer (film) that allows the substrateupon which the polysilicon circuits are formed to be released andreused. Polysilicon transistor or TFTs (Thin Film Transistor) devicesare widely used, and need not be made solely from silicon.

[0059] The various circuit layers of the 3DS memory circuit are bondedtogether by use of thermal diffusion of two metal surfaces, typicallyaluminum. The surface of the circuits to be bonded are smooth andsufficiently planar as is the case with the surface of an unprocessedsemiconductor wafer or a processed semiconductor wafer that has beenplanarized with the CMP (Chemical Mechanical Processing) method with asurface planarity of less than 1 μm and preferably less than 1,000 Åover at least the area of the surface of the circuit (formed on thesubstrate) to be bonded. The metal bonding material on the surfaces ofthe circuits to be bonded are patterned to be mirror images of eachother and to define the various vertical interconnect contacts asindicated in FIG. 2a, FIG. 2b, FIG. 2c and FIG. 5. The step of bondingtwo circuit substrates results in simultaneously forming the verticalinterconnection between the two respective circuit layers or substrates.

[0060] The thermal diffusion bonding of the circuit layers takes placepreferably in an equipment chamber with controlled pressure andatmospheric components such as N₂ with little H₂O and O₂ content. Thebonding equipment aligns the patterns of the substrates to be bonded,presses them together with a set of programmed pressures and at one ormore temperatures for a period of time as required by the type of metalused as the bonding material. The thickness of the bonding material isnominally in a range of 500 Å to 15,000 Å or greater with a preferredthickness of 1,500 Å. The initial bonding of the substrates ispreferably done at lower than standard pressure such as a negativepressure between 1 torr and 740 torr depending on the design of the bondpattern. This can leave an interior negative pressure between thebonding surfaces once external atmospheric pressure is returned whichfurther assists in the formation of the bond and enhances thereliability of the bond.

[0061] The preferred bonding material is pure aluminum or an alloy ofaluminum, but it is not limited to aluminum and may include, forexample, such metals as Sn, Ti, In, Pb, Zn, Ni, Cu, Pt, Au or alloys ofsuch metals that provide acceptable surface bond diffusion capabilitiesat acceptable temperatures and forming periods. The bonding material isnot limited to metal, and could be a combination of bonding materials,such as highly conductive polysilicon, some of which are non-conductingsuch as silicon dioxide, and the foregoing exemplary types of bondmaterial choices should not be considered to be limitations on how thecircuit layers can be bonded.

[0062] In the case where metal bond materials form a native surfaceoxide that either inhibits the forming of a satisfactory bond or mayincrease the resistance in the vertical interconnections formed by thebond, the oxide should be removed. The bonding equipment provides anoxide reduction capability such that bonding surfaces of the bondmaterial are rendered without native surface oxide. The methods offorming gas atmospheres for the reduction of surface oxides are wellknown, and there are other methods for removing the native oxide such assputter etching, plasma etching or ion mill etching. In the case wherealuminum is used as the bonding material, it is preferred that the thinnative aluminum oxide film of approximately 40 Å on the bonding surfacesbe removed prior to bonding.

[0063] The thinned (substantially flexible) substrate circuit layers ofthe 3DS memory circuit are typically memory array circuits, however, thethinned substrate circuit layers are not limited to memory circuits.Other circuit layer types can be controller circuits, non-volatilememory such as EEPROM, additional logic circuitry includingmicroprocessor logic and application specific logic functions such asthose that support graphic or database processing, etc. The selection ofsuch circuit layer types follows from the functional requirements of thedesign of the circuit and is not limited by the 3DS memory fabricationprocess.

[0064] The thinned (substantially flexible) substrate circuit layers arepreferably made with dielectrics in low stress (less than 5×10⁸dynes/cm²) such as low stress silicon dioxide and silicon nitridedielectrics as opposed to the more commonly used higher stressdielectrics of silicon oxide and silicon nitride used in conventionalmemory circuit fabrication. Such low stress dielectrics are discussed atlength in U.S. Pat. No. 5,354,695 of the present inventor, incorporatedherein by reference. The use of dielectrics with conventional stresslevels could be used in the assembly of a 3DS DRAM circuit, however, ifmore than a few layers comprise the stacked assembly, each layer in theassembly will have to be stress balanced so that the net stress of thedeposited films of a layer is less than 5×10⁸ dynes/cm². The use ofintrinsically low stress deposited films is the preferred method offabrication versus the use of the method where the stress ofindividually deposited films are not equal but are deposited to create anet balanced lower stress.

[0065] Method A, 3DS Memory Device Fabrication Sequence

[0066] This fabrication sequence assumes that several circuit layerswill be bonded to a common or support substrate and subsequently thinnedin place. An example of a resulting 3DS memory circuit is shown in FIG.1a.

[0067] 1. Align and bond to the common substrate the topside of a secondcircuit substrate.

[0068] 2A. Grind the backside or exposed surface of the second circuitsubstrate to a thickness of less than 50 μm and then polish or smooththe surface. The thinned substrate is now a substantially flexiblesubstrate.

[0069] Optionally an etch stop may be incorporated in the secondsubstrate from less than a micron to several microns below thesemiconductor surface prior to device fabrication. This etch stop can bean epitaxially formed film such as GeB (described in U.S. Pat. Nos.5,354,695 and 5,323,035 of the present inventor, incorporated herein byreference) or a low density implanted layer of O₂ or N₂ to form a buriedoxide or nitride barrier etch stop layer just below the device layer onthe topside of the second substrate. After a preliminary grinding of asignificant portion of the backside of the substrate, the remainingportion of the backside of the second substrate is then selectivelyetched in a chemical bath which stops on the surface of the eptiaxial orimplanted layer. Subsequent polishing and RIE steps as necessary canthen be used to complete the thinning of the second substrate.

[0070] Alternately, a parting layer such as H₂ implanted into thetopside surface of the second substrate prior to device fabrication canbe used with a thermal step to crack off the majority of the backside ofthe second substrate, allowing its reuse.

[0071] 2B. The second substrate may alternatively be a circuit formed ofpolysilicon transistors or TFTs over a parting layer such as aluminum,titanium, AlAs, KBr, etc. which can be activated by a specific chemicalrelease agent. The backside of the second substrate is then removed uponactivating (dissolving) the release layer and followed as needed byinterconnect semiconductor processing steps.

[0072] 3. Process the thinned backside of the second substrate to formvertical interconnections such as that shown in FIG. 4 with the bondedsurface side of the second substrate. The backside processing typicallycomprises conventional semiconductor processing steps of dielectric andmetal deposition, lithography and RIE, the order of which can vary to agreat degree. The completion of the backside processing will also resultin a patterned metal layer that is similar to the topside bond materialpattern to facilitate the subsequent bonding of an additional circuitsubstrate, a terminal pattern such as a conventional I/O IC bond pad(wire bonding) pattern, a pattern for thermal diffusion bonding of the3DS memory circuit to another die (either another 3DS circuit or aconventional die), or a pattern for insertion interconnection,conventional DCA (Direct Chip Attach) or FCA (Flip-Chip Attach).

[0073] Referring more particularly to FIG. 4, during the fabrication ofactive circuit devices, an oxide mask 401 is thermally grown ordeposited. Vertical bus contacts 403 are then formed, for example fromhighly-doped polysilicon coincident with a polysilicon gate formingstep. Alternatively, contact 403 may be formed of metal. ConventionalDRAM interconnect structures 410 are then formed using conventionalprocessing. The DRAM interconnect may include an internal pad 405. The“DRAM processed” portion 420 of the wafer includes various dielectricand metal layers. A final passivation layer 407 is deposited, afterwhich vias 409 are formed. Conventional CMP processing is then used toobtain a planar surface 411. Contacts 413 and bond surfaces not shownare then patterned in a top-most metal layer (e.g, Metal-3).

[0074] After bonding and thinning of the backside of the secondsubstrate to about 1-8 μm of silicon (or other semiconductor) substrate415, feed-throughs 417 are then formed in registration with the contacts403. A passivation layer 419 and contacts 421 are then formed. Thecontacts 421 may be formed so as to form a mirror image of the contacts413, allowing for the bonding of further wafers.

[0075] 4. If another circuit layer is to be bonded to the 3DS circuitstack, steps 1-3 are repeated.

[0076] 5A. The circuits of the finished 3DS memory substrate are thenconventionally sawed into die (singulated), resulting in a circuit ofthe type shown in FIG. 1a, and packaged as would be the case withconventional integrated circuits.

[0077] 5B. The circuits of the finished 3DS memory substrate are thenconventionally sawed and then individually aligned and thermal diffusionbonded (metal pattern down) to the surface of a second (conventional IC)die or MCM substrate in a manner similar to that used in the bonding ofthe circuit substrates of step 1 above. (The conventional die or MCMsubstrate may have a larger area than the 3DS memory substrate and mayinclude a graphics controller, video controller or microprocessor, suchthat the 3DS becomes embedded as part of another circuit.) This finalbonding step typically incorporates a fine-grain interconnect betweenthe 3DS memory circuit and the die or MCM substrate, but could also usea conventional interconnect pattern. Further, a 3DS memory circuit canbe bonded face up to a conventional IC in die form or MCM substrate andwire bonding used to form conventional I/O interconnections.

[0078] Method B, 3DS Memory Device Fabrication Sequence

[0079] This fabrication sequence assumes that a circuit substrate willfirst be bonded to a transfer substrate, thinned and then bonded to acommon substrate as a layer of the circuit stack. The transfer substrateis then released. This method has the advantage over Method A ofallowing substrates to be thinned prior to being bonded to the finalcircuit stack and allows for simultaneous thinning and verticalinterconnect processing of substrate circuit layers.

[0080] 1. Bond to a transfer substrate a second circuit substrate usinga release or parting layer. A transfer substrate may have high toleranceparallel surfaces (TTV or Total Thickness Variance of less than 1 μm)and may be perforated with an array of small holes to assist the partingprocess. The parting layer can be a blanket deposition of a bondingmetal. Precise alignment of the surfaces is not required.

[0081] 2. Perform step 2A or 2B of Method A.

[0082] 3. Process the backside of the second substrate to forminterconnections with the bonded topside surface of the second substrateas shown in FIG. 4. The backside processing typically comprisesconventional semiconductor processing steps of dielectric and metaldeposition, lithography and RIE, the order of which can vary to greatdegree. The completion of the backside processing will also result in apatterned metal layer that is similar to the bond material pattern ofthe common substrate to facilitate the subsequent bonding of anadditional circuit layer.

[0083] 4. Bond the second circuit to a common or support substrate (3DSstack) and release the transfer substrate by activating the partinglayer between it and the second circuit.

[0084] 5. Process the now exposed topside of the second substrate toform interconnections for subsequent substrate bonding or a terminalpattern for conventional I/O bonding (wire bonding) pad pattern, apattern for thermal diffusion bonding of the 3DS memory circuit toanother die (either another 3DS circuit or a conventional die), or apattern for conventional insertion interconnect, DCA (Direct ChipAttach) or FCA (Flip-Chip Attach). If another circuit layer is to bebonded to the 3DS circuit stack, steps 1 through 4 are repeated.

[0085] 6. Perform step 5A or 5B of Method A.

[0086] 3DS Memory Device Yield Enhancement Methods

[0087] The 3DS circuit may be considered a vertically assembled MCM(Multi-Chip Module) and as with an MCM the final yield is the product ofthe yield probabilities of each component circuit (layer) in thecompleted 3DS circuit. The 3DS circuit uses several yield enhancementmethods that are synergistic in their combined usage within a singlememory IC. The yield enhancement methods used in the 3DS memory circuitinclude small memory array block size, memory array block electricalisolation through physically unique or separate vertical businterconnections, intra memory array block gate-line sparing, memoryarray layer sparing (inter-block gate-line sparing), controller sparingand ECC (Error Correcting Codes). The term sparing is used to meansubstitution by a redundant element The selected size of the memoryarray block is the first component in the yield equation for the 3DSmemory circuit. Each memory array block is individually (uniquely)accessed and powered by the controller circuit and is physicallyindependent of each and every other memory array block including thoseon the same memory array layer in addition to those on a differentmemory array layer. The size of the memory array block is typically lessthan 5 mm² and preferably less than 3 mm², but is not limited to aspecific size. The size of memory array block, the simplicity of itsNMOS or PMOS fabrication process and its physical independence from eachof the other memory array blocks, for nearly all production ICprocesses, provides a conservatively stated nominal yield of greaterthan 99.5%. This yield assumes that most point defects in the memoryarray block such as open or shorted interconnect lines or failed memorycells can be spared (replaced) from the intra-block or inter-block setof redundant gate-lines. Major defects in a memory array block whichrender the complete memory array block unusable result in the completesparing of the block from a redundant memory array layer or therejection of the 3DS circuit.

[0088] In the example of a 3DS DRAM circuit the yield of a stack ofmemory array blocks is calculated from the yield equationYs=((1−(1−P_(y))²)^(n))^(b), where n is the number DRAM array layers, bis the number of blocks per DRAM array and P_(y) is the effective yield(probability) of a DRAM array block less than 3 mm² in area. Assuming aDRAM array block redundancy of 4% for gate-lines in the DRAM array blocklines and one redundant DRAM array layer, and assuming further that thenumber of blocks per layer is 64, the number of memory array layers inthe stack is 17 and the effective value for Py is 0.995, then the stackyield Ys for the complete memory array (including all memory array blockstacks) is 97.47%.

[0089] The Ys memory array stack yield is then multiplied by the yieldof the controller Yc. Assuming a die size of less than 50 mm², areasonable Yc for a controller fabricated from a 0.5 μm BiCMOS or mixedsignal process would be between 65% and 85%, giving a net 3DS memorycircuit yield of between 63.4% and 82.8%. If a redundant controllercircuit layer is added to the 3DS memory stack, the yield probabilitieswould be between 85.7% and 95.2%.

[0090] The effective yield of a memory array block can be furtherincreased by the optional use of ECC logic. ECC logic corrects data biterrors for some group size of data bits. The syndrome bits necessary forthe operation of ECC logic would be stored on redundant gate-lines ofany of the memory array layers in a vertically associated block stack.Further, if necessary, in order to accommodate the storage of ECCsyndrome bits, additional memory array layers could be added to thecircuit.

[0091] Advantageous 3DS Memory Device Controller Capabilities

[0092] As compared to a conventional memory circuit, the 3DS memorycontroller circuit can have various advantageous capabilities due theadditional area available for controller circuitry and the availabilityof various mixed signal process fabrication technologies. Some of thesecapabilities are self-test of memory cells with dynamic gate-lineaddress assignment, virtual address translation, programmable addresswindowing or mapping, ECC, data compression and multi-level storage.

[0093] Dynamic gate-line address assignment is the use of programmablegates to enable the layer and gate-line for a read/write operation. Thisallows the physical order of memory storage to be separate or differentfrom the logical order of stored memory.

[0094] The testing of each generation of memory devices has resulted insignificantly increased test costs. The 3DS memory controller reducesthe cost of testing by incorporating sufficient control logic to performan internal test (self-test) of the various memory array blocks. Circuittesting in the conventional ATE manner is required only for verificationof controller circuit functions. The scope of the internal test isfurther extended to the programmable (dynamic) assignment of uniqueaddresses corresponding to the various gate-lines of each memory arrayblock on each layer. Self-test capability of the 3DS controller circuitcan be used anytime during the life of the 3DS memory circuit as adiagnostic tool and as a means to increase circuit reliability byreconfiguring (sparing) the addresses of gate-lines that fail after the3DS memory circuit is in use in a product.

[0095] ECC is a circuit capability that, if included in the controllercircuit, can be enabled or disabled by a programming signal or made adedicated function.

[0096] Data compression logic will allow the total amount of data thatcan be stored in the 3DS memory array to be increased. There are variousgenerally known data compression methods available for this purpose.

[0097] Larger sense amps allow greater dynamic performance and enablehigher speed read operations from the memory cells. Larger sense ampsare expected to provide the capability to store more than one bit(multi-level storage) of information in each memory cell; thiscapability has already been demonstrated in nonvolatile memory circuitssuch as flash EPROM. Multi-level storage has also been proposed for usein the 4 Gbit DRAM generation circuits.

[0098] It will be appreciated by those of ordinary skill in the art thatthe invention can be embodied in other specific forms without departingfrom the spirit or essential character thereof. The presently disclosedembodiments are therefore considered in all respects to be illustrativeand not restrictive. The scope of the invention is indicated by theappended claims rather than the foregoing description, and all changeswhich come within the meaning and range of equivalents thereof areintended to be embraced therein.

What is claimed is:
 1. A method of forming a random-access memory,comprising the steps of: fabricating a memory circuit on a firstsubstrate; fabricating a memory controller circuit on a secondsubstrate; and bonding the first and second substrates to forminterconnects between the memory circuit and the memory controllercircuit, neither the first substrate alone nor the second substratealone being sufficient to provide random access data storage.
 2. Themethod of claim 1, wherein said bonding is thermal diffusion bonding ofthe first substrate to the second substrate to form a stacked ICstructure.
 3. The method of claim 2, wherein at least some of saidinterconnects are fine-grain vertical interconnects having a pitch ofless than 100 μm.
 4. The method of claim 3, comprising the step offurther bonding the stacked IC structure and a further substrate.
 5. Themethod of claim 4, wherein said further bonding is thermal diffusionbonding of the stacked IC structure and the further substrate to oneanother.
 6. The method of claim 5, wherein said thermal diffusionbonding uses fine-grain contact patterns having a contact pitch of lessthan 100 μm.
 7. The method of claim 6, wherein said fine-grain contactpatterns form extensions of the fine-grain vertical interconnects. 8.The method of claim 1, comprising the step of further bonding thestacked IC structure and a further substrate.
 9. The method of claim 8,wherein said further bonding is wire bonding of a singulated stacked ICstructure and the further substrate.
 10. The method of claim 1, whereinat least some of the interconnects are formed by a planar process. 11.The method of claim 8, wherein said further bonding is thermal diffusionmetal bonding of fine-grain vertical interconnect contact pattern of asingulated stacked IC structure and the further stacked IC orconventional circuit IC.
 12. The method of claim 8, wherein said furtherbonding is thermal diffusion metal bonding of an interconnect contactpattern of a singulated stacked IC structure and the further stacked ICor conventional circuit IC.
 13. The method of claim 11, wherein thefirst and second substrates are wire bonded to a third substrate. 14.The method of claim 1, wherein said bonding is thermal diffusion bondingof the first substrate to the second substrate to form a stacked ICstructure, the method comprising the further steps of: fabricating atleast one additional memory circuit on at least one additionalsubstrate; and bonding the at least one additional substrate to thestacked IC substrate and forming interconnects between the at least oneadditional memory circuit and the memory controller circuit, wherein atleast some of the interconnects pass through a substrate on which amemory circuit is formed.
 15. The method of claim 14, further comprisingthe step of: thinning substrates on which memory circuits are formed toform thinned substrates, facilitating formation of said interconnects.16. The method of claim 15, wherein at least some of said interconnectsare fine-grain vertical interconnects having a pitch of less than 100μm.
 17. The method of claim 15, wherein said thinned substrates arethinned to less than 50 μm in thickness.
 18. The method of claim 15,wherein semiconductor portions of the thinned substrates are thinned toa thickness in the range of about 1-8 μm.
 19. The method of claim 15,wherein said thinning comprises grinding said substrates.
 20. The methodof claim 19, wherein said substrates are ground subsequent to beingbonded.
 21. The method of claim 19, wherein said substrates are groundprior to being bonded.
 22. The method of claim 14, wherein at least onememory circuit is formed on a reusable substrate, further comprising thestep of separating a layer in which the memory circuit is formed fromthe reusable substrate.
 23. The method of claim 22, wherein the at leastone memory circuit is formed of polysilicon transistors.
 24. The methodof claim 14, wherein bonding comprises thermal diffusion bonding. 25.The method of claim 24, wherein mating contact patterns are formed onrespective surfaces to be bonded together.
 26. The method of claim 25,wherein said mating contact patterns are formed predominantly of metal.27. The method of claim 26, wherein said metal includes metal selectedfrom a group consisting of: Al, Sn, Ti, In, Pb, Zn, Ni, Cu, Pt and Au,and alloys thereof.
 28. The method of claim 14, wherein said memorycircuits and said memory controller circuit are semiconductor circuits,and wherein the memory controller circuit is fabricated using a firstsemiconductor process technology and the memory circuits are formedusing a second distinct semiconductor process technology.
 29. The methodof claim 28, wherein the first semiconductor process technology employsactive semiconductor devices of both a first type and a secondcomplementary type.
 30. The method of claim 28, wherein semiconductordevices formed in accordance with the second semiconductor processtechnology include MOS semiconductor devices, the MOS semiconductordevices all being of a single type.
 31. A method of informationprocessing using a stacked integrated circuit memory including a memorycontroller layer and multiple memory layers, the method comprising thesteps of: initiating a memory access; and independently routing datavertically between the memory controller layer and selected storagelocations within each of a plurality of equal-size memory blocks. 32.The method of claim 31, comprising the further steps of: during a singlememory access, accessing data from multiple memory layers.
 33. Themethod of claim 32, wherein data from one memory layer is used insteadof data from another memory layer having a defective portion.
 34. Themethod of claim 32, wherein data from one memory layer is used toperform ECC processing with respect to data from another memory layer.35. The method of claim 31, comprising the further steps of: receivingwithin the memory controller layer data from the selected storagelocations; and for each selected storage location, distinguishingbetween at least four voltage levels to produce at least two bits ofdata.
 36. The method of claim 31, comprising the further steps of:receiving data within the memory controller layer; and decompressing thedata.
 37. The method of claim 31, comprising the further steps of:compressing data within the memory controller layer; and writing thedata to the selected memory locations.
 38. A stacked integrated circuitmemory comprising: a first substantially rigid substrate having formedthereon one of a memory circuit and a memory controller circuit; and atleast one substantially flexible substrate having formed thereon theother of said memory circuit and said memory controller circuit andbeing bonded to the first substrate.
 39. The apparatus of claim 38,wherein the first substrate has formed thereon the memory circuit and ispart of a stack of memory circuit substrates, and the second substratehas formed thereon the memory controller circuit.
 40. The method ofclaim 39, wherein the first and second substrates are singulated die,the second substrate having greater area than the first substrate. 41.The method of claim 40, wherein the second substrate has formed thereonadditional circuitry separate from the memory controller circuit. 42.The method of claim 41, wherein the additional circuitry is part of agraphics display subsystem.
 43. The method of claim 41, wherein theadditional circuitry comprises a microprocessor.
 44. The apparatus ofclaim 38, wherein the substantially flexible substrate includes memoryI/O pads.
 45. The apparatus of claim 44, wherein the memory circuit isformed proximate a top surface of the flexible substrate, the topsurface being bonded to the first substrate, and the memory I/O pads areformed proximate an opposite bottom surface of the flexible substrate.46. The apparatus of claim 38, wherein the memory circuit and the memorycontroller circuit are coupled by vertical interconnects.
 47. Theapparatus of claim 46, wherein said vertical interconnects includefine-grain vertical interconnects formed at a pitch of less than 100 μm.48. The method of claim 47, wherein at least some of the fine-grainvertical interconnects are arrayed in two dimensions.
 49. The apparatusof claim 47, wherein said memory circuit comprises a two-dimensionalarray of memory blocks, each memory block having formed proximatethereto an array of fine-grain vertical interconnects forming a firstport coupling the memory block to the memory controller.
 50. Theapparatus of claim 49, wherein at least some memory blocks have formedproximate thereto an array of fine-grain vertical interconnects forminga second port coupling the memory block to the memory controller. 51.The apparatus of claim 38, wherein at least one of said memory circuitsprovides redundant memory locations.
 52. The apparatus of claim 51,further comprising an additional substantially flexible substrate onwhich is formed a redundant memory circuit.
 53. The apparatus of claim52, wherein the memory controller circuit comprises ECC logic and isprogrammed to store ECC syndromes in the redundant memory circuit. 54.The apparatus of claim 51, wherein the memory controller circuitcomprises logic for testing the memory circuit.
 55. The apparatus ofclaim 54, wherein the memory controller circuit is programmed tosubstitute redundant memory locations for defective memory locations inthe memory circuit.
 56. The apparatus of claim 38, wherein the memorycontroller circuit comprises logic for performing at least one of thefollowing functions: virtual memory management, indirect addressing,content addressing, data compression, data decompression, graphicsacceleration, audio encoding, audio decoding, video encoding, videodecoding, voice recognition, handwriting recognition, power management,and database processing.
 57. The apparatus of claim 38, furthercomprising a second substrate having formed thereon a redundant memorycontroller, bonded to the substantially flexible substrate.
 58. Theapparatus of claim 38, further comprising a second substrate havingformed thereon a microprocessor, bonded to the substantially flexiblesubstrate.
 59. The apparatus of claim 38, wherein the memory controllercircuit comprises sense amplifiers coupled to data lines of the memorycircuit.
 60. The apparatus of claim 59, wherein the sense amplifiersdiscriminate between more than two signal levels, each sense amplifierproducing a multilevel output signal.
 61. The apparatus of claim 59,wherein the sense amplifiers are sized so as to exhibit a switchingspeed of about 10 ns or less.
 62. A method of bonding together multiplesubstrates each having integrated circuits formed thereon to forminterconnections between the integrated circuits, the method comprisingthe steps of: processing a mating surface on each of first and secondsubstrates to achieve substantial planarity of the mating surfaces;forming mating, fine-grain interconnect patterns on the mating surfaces;and performing fine-grain, planar thermal diffusion bonding of themating surfaces.
 63. The method of claim 62, wherein said thermaldiffusion bonding of the first substrate to the second substrate forms astacked IC structure.
 64. The method of claim 63, wherein at least someof said interconnects are fine-grain vertical interconnects having apitch of less than 100 μm.
 65. The method of claim 64, comprising thestep of further bonding the stacked IC structure and a furthersubstrate.
 66. The method of claim 65, wherein said further bonding isthermal diffusion bonding of the stacked IC structure and the furthersubstrate to one another.
 67. The method of claim 66, wherein saidthermal diffusion bonding uses fine-grain contact patterns having acontact pitch of less than 100 μm.
 68. The method of claim 67, whereinsaid fine-grain contact patterns form extensions of the fine-grainvertical interconnects.
 69. The method of claim 62, comprising the stepof further bonding the stacked IC structure and a further substrate. 70.The method of claim 69, wherein said further bonding is wire bonding ofa singulated stacked IC structure and the further substrate.
 71. Themethod of claim 62, wherein at least some of the interconnects areformed by a planar process.
 72. The method of claim 69, wherein saidfurther bonding is thermal diffusion metal bonding of fine-grainvertical interconnect contact pattern of a singulated stacked ICstructure and the further stacked IC or conventional circuit IC.
 73. Themethod of claim 69, wherein said further bonding is thermal diffusionmetal bonding of an interconnect contact pattern of a singulated stackedIC structure and the further stacked IC or conventional circuit IC. 74.The method of claim 72, wherein the first and second substrates are wirebonded to a third substrate.
 75. The method of claim 62, furthercomprising the step of: thinning said substrates on which saidintegrated circuits are formed to form thinned substrates, facilitatingformation of said interconnects.
 76. The method of claim 75, wherein atleast some of said interconnects are fine-grain vertical interconnectshaving a pitch of less than 100 μm.
 77. The method of claim 75, whereinsaid thinned substrates are thinned to less than 50 μm in thickness. 78.The method of claim 75, wherein semiconductor portions of the thinnedsubstrates are thinned to a thickness in the range of about 1-8 μm. 79.The method of claim 75, wherein said thinning comprises grinding saidsubstrates.
 80. The method of claim 79, wherein said substrates areground subsequent to being bonded.
 81. The method of claim 79, whereinsaid substrates are ground prior to being bonded.
 82. The method ofclaim 74, wherein at least one integrated circuit is formed on areusable substrate, further comprising the step of separating a layer inwhich the integrated circuit is formed from the reusable substrate. 83.The method of claim 82, wherein the at least one integrated circuit isformed of polysilicon transistors.
 84. The method of claim 74, whereinbonding comprises thermal diffusion bonding.
 85. The method of claim 84,wherein mating contact patterns are formed on respective surfaces to bebonded together.
 86. The method of claim 85, wherein said mating contactpatterns are formed predominantly of metal.
 87. The method of claim 86,wherein said metal includes metal selected from a group consisting of:Al, Sn, Ti, In, Pb, Zn, Ni, Cu, Pt and Au, and alloys thereof.